An increased use of superconducting generators and motors will occur for a number of reasons. Generator ratings are increasing making superconducting efficiency more important. Applications, particularly for wind power, include land-based, offshore and remote locations. Present technologies, such as the use of high speed iron gearboxes, are problematic because the gearboxes are too unreliable and heavy. Direct drive machines are becoming popular because they do not have gearboxes, but these are very heavy and difficult to transport and install.
Direct drive superconducting generators are potentially lighter, but the topology associated with wind generation makes current proposed superconducting wind generator systems less practical. Present superconducting designs generally utilize an old-style single cryostat and a superconducting rotor in which all components are cold. They also feature large and fragile torque tubes. These units suffer many single point failures.
These units must be built and shipped and installed as a single piece. The rotating seals isolating the cryostat are problematic, especially as traditional topology is not well suited for harsh and unattended environments. Traditional superconducting materials, such as yttrium barium copper oxide (YBCO) and bismuth strontium calcium copper oxide (BSCCO) conductors are very expensive. Any failure effectively kills the machine and requires that an entire machine be removed, shipped to a factory, repaired and reinstalled; every step of which results in a very expensive process. The associated long down-time, high cost, make such installation economically unviable.
Consequently, a new technology has to be developed that allows 5-20 MW (megawatt) generators that solve these main problems of large direct drive superconducting generators. The present invention, among other advantages, addresses all of these issues with a lower cost, highly reliable, repairable on site, easily transported, installed and repaired superconducting generator that meets the needs of high power (>5 MW wind energy systems) installations. These ends are accomplished in part without a single large cryostat with fragile structure and in which single point failures can be catastrophic. The present embodiments of the invention do not use a complex and large cryostat/torque tube design. There are no single large cold areas, just cool areas that need to be cryogenic (i.e. coils); where redundancy means that the installation can lose a coil, or even two, and generator can keep running. There is an elimination of many problem areas including a single big cryostat, torque tube, and the introduction of multiple redundant cryostats. Further, the design employs multiple topology improvements to improve manufacturing, transportation, installation, and repair.
The existing state of the art for non-superconducting wind generators includes high speed generators which are relatively light, and gear boxes that are heavy and unreliable; and direct drive conventional generators are heavy, but reliable. These large gear boxes and large direct drive generators are limited in their ultimate rating on wind towers and pose transportation, assembly and repair issues, especially in regards to remote land and/or offshore locations. Direct drive superconducting generators will be potentially lighter than conventional units (high speed generator and gear boxes), but they are still big and heavy with regard to transportation over highways in the 5-20 MW rating. Present traditional superconducting designs can be unreliable and prone to single point failures, hard to repair and hard to transport and install.
What are needed are direct drive superconducting generators with new topology that overcome traditional superconducting generators' problems and which are well suited to wind applications. The superconducting generator can be used as direct drive without a gear box, or can also be used with a highly simplified one stage gearbox that yields high reliability and is lightweight.
High Power Wind Platforms (5-20 MW) will offer significant economies of scale and may be required to make wind energy economically viable. At present, many wind platforms use high speed (1200 to 1800 rpm) generators driven through large, heavy, and historically very unreliable, multistage gearboxes. Direct drive (low rpm) generators are being explored to eliminate the highly unreliable gearboxes, but this results in very large and very heavy generators, and the problem promises to get much worse as wind platform outputs exceed 5-6 MW and approach 10-20 MW. The large size and high weight of proposed conventional (iron-based, non-superconducting) direct drive generators may make proposed high power wind platforms logistically and economically impractical.
There are significant logistical issues associated with platforms rated above 4 MW for land systems and 8 MW for offshore systems. Economies of scale suggest that wind platforms may be more economically attractive if their ratings can be pushed to 5-20 MW.
High speed multi-megawatt generators can be of a manageable size and weight but the relatively large and heavy gearboxes needed to drive the generators are proving to be very costly, unreliable and expensive to maintain, and are the key limiting factor to the economic viability of multi-megawatt wind systems. This problem will become much worse as the output of these emerging systems exceeds 5-8 MW and may prevent the commercialization of high power wind systems. In order to eliminate the gearboxes, direct drive systems are being proposed, but the large size and weight of low speed (6-12 rpm) generators make this equipment very difficult to transport and assemble in off-shore or remote land locations. The towers needed to support these heavy generators are also difficult and very expensive to build, again especially in remote land or offshore locations. Accordingly, size, weight and reliability issues along with transportation, assembly and maintenance issues may prevent the commercialization of high power (5-20 MW) wind systems without further technological advances.
A high speed traditional superconducting generator with a gearbox would have some weight advantages, but would pose reliability issues with the generator and gearbox. A low speed direct drive conventional generator (no gearbox) might be reliable but would also likely be very large and heavy, thus posing numerous transportation, installation and repair issues. A low speed direct drive traditional superconducting generator would be lighter, but still large, and again would entail transportation, installation and repair issues and generator reliability concerns. The low speed direct drive superconducting generator proposed herein would be significantly lighter, have minimized transportation, installation and repair issues, as well as having a more reliable generator. The mid-speed superconducting generator with a single stage gearbox, also proposed herein, would also be a light weight system, with minimized transportation, installation and repair issues, and would also have a high degree of reliability.
Generator Power and Power Density Generator power outputs are governed by the equation:Generator Power=Const*D2*L*B*A*rpm, where:D=Rotor DiameterL=Rotor Active Length B=Magnetic FieldA=Stator Current DensityRpm=Rotor Speed (rpm)
In a comparison of high speed vs. low speed generators, using traditional iron-based technology, flux densities are the same (2 Tesla) and stator current densities are the same, and limited by cooling technology in copper conductors. Although low speed generators can be larger in diameter than high speed machines, this increased diameter does not make up for the low speed.
Traditional high-speed iron-based generators are well proven with reliable technology, as seen in utilities and commercial systems, and are reasonably small and of moderate weight. As said, the technology is proven and readily extendable to tens or thousands of MWS. High-speed (1,800 rpm and above) iron-based generators can be small, lightweight, transportable and reliable and would be the system of choice if they did not require a gearbox to increase speed.
The issues surrounding speed increasing for multi-megawatt gearboxes are complex. High power speed-decreasing gearboxes are relatively large and heavy, however, they are at least fairly reliable, for example, as seen in ship drives and other present applications. Proposed wind systems, however require multi-MW multi-stage speed increasing gearboxes which are large, heavy and very unreliable. They are responsible for the bulk of systems failures and are difficult and expensive to maintain. A single stage gearbox design, however, decreases reliability and maintenance issues and results in large weight reduction. “High-Speed” (1800 rpm) generators are relatively lightweight but they require large and heavy gearboxes that are extremely unreliable and are, in fact, proving problematic as power levels increase. Direct drive (10 rpm) generators eliminate the troublesome gearbox, but these generators are very large and heavy, a major problem for shipping installation, repair and tower support requirements. These problems are intensified as power levels increase.
As an interesting alternative to direct drive; a “Hybrid Drive” (Single Stage 4:1 Gearbox and a 40 rpm superconducting generator) offers great advantages. The generator is significantly smaller and lighter and single stage gearbox is lighter and can be highly reliable.
Disadvantages of Present Superconducting Wind Generators
Present superconducting wire generators pose a number of problems. The conductors themselves are fragile, when compared to copper. Fragile, complex hardware is needed to support and cool conductors to extremely low temperatures, such as cryostats, torque tubes, cryocoolers, and rotating cryogenic seals. Cryogenic rotors cannot be repaired on site and have many single point failure mechanisms. These machines tend to be unreliable, virtually impossible to repair on site and must be transported and installed as a single unit, making them large and heavy, and therefore very difficult to transport and install in remote locations on land or offshore. Traditional superconducting generator designs have unproven reliability, especially in harsh environments.
Some superconducting generator designs being developed for direct drive wind platforms have adopted many of the traditional superconducting generator topologies developed for previous land-based high speed superconducting generators. These topologies are large, heavy and ineffective for the needs of emerging high power wind energy platforms. A new topology is needed that specifically addresses the previously discussed issues.
Specific Problems with Presently Evolving Superconducting Generators
One traditional “One Big Cryostat” design enclosing the entire rotor and mechanism, including an entire internal structure, can be operated at cryogenic temperatures. It is susceptible to major heat leak and the large cryostat has to be shielded from eddy losses. The large structure is vulnerable to small pinhole leaks and cracks in its very large structure that will completely disable the entire machine. Even a small leak will likely require that the generator be removed from the platform, returned to the manufacturer, repaired, returned and reinstalled. This consumes a great deal of time, money and downtime.
Traditional torque tubes are not well suited for very high torque, low speed, generators with large pulsating loads. They are traditionally used in high speed applications where torque is relatively small and in lower power machines where the torque tube is small. It can be very complex to absolutely minimize the heat leak through the torque tube.
Cylindrical rotors are, typically used on small diameter machines (10 inches to 4 feet) due to the reasonable proportions required. Wind generators, however, will be very large in diameter and have a large amount of relatively empty space in the center. A torque tube for these machines will be very large in diameter and have a great deal of weight and material and potential heat leak, also it will add significantly to the overall length of the rotor and therefore to the generator. They are fragile and have significant heat leak, significant added length and significantly reduced stiffness of the shaft.
A New Design Approach to Superconducting Generators in Wind Platforms
A unique superconducting coil configuration may have a plurality of small cryostats. Only the superconducting coils are cold and each is enclosed in its own small cryostat. There is less surface area and therefore heat leak than with a traditional large cryostat and torque tube, with a resulting lower cooling requirement and need for fewer cryocoolers. The design may have one-half the number of coils (1 per pole pair), and again fewer cryostats lower the cooling requirements. A wavy wound stator winding allows the generator to function even if one or even two rotor coils or cryostats fail, thus promoting high reliability and far less expensive maintenance.
Coils and cryogenic components can be replaced on-site quickly and inexpensively and their high redundancy and reliability minimize traditional single point failures. The design promotes easy on-site repair and replacement of most critical generator parts (such as cryogenic rotor parts). This approach will also work for all cryogenic machines where the stator is also superconducting. The plurality of cryostats are small and more robust and reliable, and designed to be easily replaced or repaired on-site with small interchangeable modules. A small inventory of interchangeable parts greatly reduces repair and maintenance costs and downtimes. Again, the redundancy of a wave wound stator winding allows the generator to continue to function even if a rotor coil fails, a significant benefit and one not possible with traditional superconducting generators.
Generators in Wind Platforms: Cooling Systems and Coolant
Each of the plurality of redundant cryostats has a small required capacity. It is possible to have rotating cryocoolers to provide coolant to the cryostats, thereby eliminating chronic problems found with rotating cryogenic seals. Liquid hydrogen (LH2) might prove to be a superior coolant, as some superconductors work well at 20 K, and LH2 is cheap and easily replenished on-site, which is also a factor driving the use of magnesium diboride (MgB2), as liquid helium is developing a shortage issue.
The design has no large vulnerable single cryostat and no large, complex and vulnerable torque tube. The cryosystem cools only the coils, not the entire structure. There are lower cooling requirements since the area of small cryostats has less surface area and cooling requirements than that of traditional big cryostat. The rotor and stator may be built in small lightweight segments, enhancing shipping installation and allowing fast on-site repairs. A modular design allows for replacement of critical parts from small inventories of standard parts and the rotor design eliminates many traditional failure points and provides redundancy.
The design may have one-half the number of rotor coils, because one superconducting coil could drive two poles in the generator, which could mean one-half the risk of cryostat failure and the wavy wound stator windings can allow the generator to continue to operate with one and possibly two rotor coil failures. It may be possible to eliminate rotating seals by mounting cryocoolers on the rotor with lockup seals for recharging the system. YBCO is unlikely to be economical or available in sufficient quantities or long enough lengths, and BSCCO remains very expensive, thus making MgB2 a likely choice.
Superconducting Generators Will be Needed as Ratings Increase Above 5 MW
While non-superconducting designs may be designed to work on tall towers and long blades; the difficult task of minimizing weight on such towers will be essential as ratings increase. A proposed single stage gearbox (4:1) helps reduce weight. For a large MW machine, the gearbox weight can be reduced by incorporating the gearbox into the end bell housing of generators.